Although not limited to such a use, the electrolytic stripping cell of the invention is, as stated, particularly adapted for use in processing silver brazed, stainless steel honeycomb scrap. Such scrap is commonly found in aircraft. For example, General Dynamics B-58, Lockheed C-141, and McDonnell F-4 aircraft contain honeycomb panels and sections which are to be removed and processed prior to processing the remainder of the airplanes for scrap.
It has been determined that silver braze can be electrolytically stripped from such shredded stainless steel honeycomb, and U.S. Patent Application Ser. No. 814,959, filed on July 12, 1977, now U.S. Pat. No. 4,090,935, granted May 23, 1978, discloses a suitable process. However, it has been found that commercially available stripping cells are not suitable for this purpose.
Well known commercial cells include the commercial Moebius type and Balbach-Thum cell systems. These cell systems are described for example in Silver, Economics, Metallurgy and Use, Butts and Coxe, D. Van Nostrand Company, Inc., Princeton, New Jersey 1967, p. 86. Both systems use a silver nitrate/copper nitrate electrolyte, a stainless steel or graphite cathode for deposition of the silver crystals, an impure Dore anode that assays between 94 to 99 percent silver, and wiper blades or scrapers to break off the silver dendrites and prevent electrical shorts between cathode and anode. The bulk density of the cast Dore anode would approximate 10.6 grams per cubic centimeter so the anode basket or compartment necessary to contain the Dore anode in these cells is relatively small.
Either of these commercial cell systems would have decided disadvantages in processing shredded, silver brazed, stainless steel honeycomb. For example, the small anode basket size presents problems. Specifically, the bulk density of the shredded, silver brazed, stainless steel honeycomb ranges between 0.4 to 0.8 grams per cubic centimeter compared to 10.6 grams per cubic centimeters for Dore anodes. A second related factor is the percentage of silver in the two anode charges. An anode charge of shredded, silver brazed, stainless steel honeycomb, such as being considered here, assays approximately 10 percent silver whereas the Dore anode is nominally 95 percent silver. These two factors, low bulk density and low silver content, require the anode basket to be the largest item, with regard to volume, in the cell for the cell to be practical commercially. Further, the lack of electrolyte circulation in the two commercial electrolytic cell systems is a decided disadvantage. In this regard, it has been determined in electrolytic stripping tests run for a reasonable length of time, and without rather vigorous electrolyte circulation, that silver recovery is less than 60 percent. In addition, chemical assays of the residual shredded stainless steel honeycomb removed from simulated Thum cell tests indicated that pieces of the outer skin detached from the honeycomb material by the shredding action was virtually free of silver; however, pieces of the cellular core structure still attached to outer pieces of skin contained recoverable amounts of silver. It will be appreciated that in a situation where each silver ion is made to travel a very complex and intricate path between the anode and cathode through a maze of inert baffling, agitation or stirring of the electrolyte is necessary even in commercial cells. Thus, when shredded, silver brazed, stainless steel honeycomb is used as the anode in an electrolytic silver refining cell, an analogous situation exists, i.e., a very complex path is provided, and commercial cells, which do not provide such stirring, have been found to be impractical.
Other prior art of possible interest includes U.S. Pat. Nos. 2,563,903 (Zadra); 3,715,229 (Anderson et al); 3,915,832 (Backus et al); and 3,985,634 (Larson et al).